The use of ultrasound for destroying predetermined groups of tissue cells, such as tumors, is known. See, for example U.S. Pat. Nos. 3,958,559, 3,237,623 and 3,117,571. Typically, a piezoelectric ultrasonic generator driven by an rf amplifier is coupled to an ultrasonic lens of known focal length. The locus of the cells to be destroyed is first determined by employing, for example, known pulse-echo imaging techniques. During this phase, the ultrasonic transducer is driven at a sufficiently low intensity to preclude tissue cell damage. Once the locus of the target cells is fixed, the lens is focused on the target area, as by means of a stereotaxic device, and the intensity of the ultrasound is increased to a level sufficient to effect tissue destruction by thermal heating.
Because this technique relies on thermal heating to destroy the target cells, it is necessary to precisely focus the ultrasonic beam to avoid damage to surrounding tissue cells. For example, when the target area is in the brain, imprecise focusing can cause extensive damage. However, precise focusing is not always possible, as where the target area is smaller than the focal zone of the lens, or where the target cells are transient, as in the case of leukemia.
Another known ultasonic technique involves the direct application of ultrasound to the target area by means of a vibrating surgical instrument, often in the form of a probe. Where internal cells are to be destroyed, the probe must be surgically inserted into the patient to accomplish direct application, whereupon the instrument is vibrated with sufficient intensity to destroy the target cells. See, for example, U.S. Pat. Nos. 4,136,700, 4,063,557, 3,896,811, 3,823,717, 3,805,787, 3,589,363, 3,565,062 and 3,352,303. Like the focused ultrasonic beam technique described above, the use of vibrating probes is accompanied by the risk of damage to surrounding tissue cells, especially when the target area is small. Furthermore, the vibrating probe is useless in the case of transient target cells and is disadvantageous insofar as it requires surgical insertion to destroy internal cell groups.
U.S. Pat. Nos. 3,499,436 and 3,499,437 are of interest insofar as they disclose means for transmitting sonic energy to an organism for analgesic and other purposes.